I must beg leave, before I finish this chapter, to enter a caveat in the breast of my fair reader; and it is this: Not to take it absolutely for granted, from an unguarded word or two which I have dropped it, "that I am a married man."
-- Laurence Sterne, The Life and Opinions of Tristram Shandy, Gentleman, 1759
So the owner of a balky horse would issue the caveat, "You'll be surprised to see the way he works," the surprise being that he would not work at all.
-- Roger Welsch, Mister, You Got Yourself a Horse: Tales of Old-Time Horse Trading, 1987
Caveat comes from the Latin caveat, "let him beware," from cavere, "to beware."